Converter circuit for switching a large number of switching voltage levels

ABSTRACT

A converter circuit is specified for switching a large number of switching voltage levels, which has n first switching groups for each phase, with the n-th first switching group being formed by a first power semiconductor switch and a second power semiconductor switch, and with the first first switching group to the-th switching group each being formed by a first power semiconductor switch and a second power semiconductor switch and by a capacitor, which is connected to the first and second power semiconductor switches, with each of the n first switching groups being connected in series to the respectively adjacent first switching group, and with the first and the second power semiconductor switches in the first first switching group being connected to one another. In order to reduce the amount of electrical energy stored in the converter circuit, n≧1 and p second switching groups and p third switching groups are provided, which are each formed by a first power semiconductor switch and a second power semiconductor switch and by a capacitor which is connected to the first and second power semiconductor switches, where p≧1 and each of the p second switching groups is connected in series with the respectively adjacent second switching group, and each of the p third switching groups is connected in series with the respectively adjacent third switching group, and the first second switching group is connected to the first power semiconductor switch in the n-th first switching group, and the first third switching group is connected to the second power semiconductor switch in the n-th first switching group. Furthermore, the capacitor in the p-th second switching group is connected in series with the capacitor in the p-th third switching group.

DESCRIPTION

1. Technical Field

The invention relates to the field of power electronics and is based ona converter circuit for switching a large number of switching voltagelevels, as claimed in the precharacterizing clause of the independentclaim.

2. Prior Art

Nowadays, converter circuits are used in a wide range ofpower-electronic applications. The requirements for a converter circuitsuch as this are in this case firstly to produce as few harmonics aspossible on phases of an electrical AC voltage network which is normallyconnected to the converter circuit, and on the other hand to transmitpower levels that are as high as possible with the smallest possiblenumber of electronic components. One suitable converter circuit forswitching a large number of switching voltage levels is specified in DE692 05 413 T2. In this document, n first switching groups are providedfor each phase, with the n-th first switching group being formed by afirst power semiconductor switch and a second power semiconductorswitch, and the first first switching group to the (n−1)-th switchinggroup each being formed by a first power semiconductor switch and asecond power semiconductor switch and by a capacitor which is connectedto the first and to the second power semiconductor switch, where n≧2.Each of the n first switching groups is connected in series with therespectively adjacent first switching group, with the first and thesecond power semiconductor switches in the first first switching groupbeing connected to one another. The first and the second powersemiconductor switches are in each case formed by an insulated gatebipolar transistor (IGBT) and by a diode connected back-to-back inparallel with the bipolar transistor.

A converter circuit for switching a large number of switching voltagelevels according to DE 692 05 413 T2 is subject to the problem that theamount of electrical energy stored in the converter circuit duringoperation is very high. Since the electrical energy is stored in thecapacitors in the n first switching groups of the converter circuit, thecapacitors must be designed for this electrical energy, that is to sayin terms of their withstand voltage and/or their capacitance. However,this necessitates capacitors with a large physical size, which arecorrespondingly expensive. Furthermore, because the physical size of thecapacitors is large, the converter circuit requires a large amount ofspace, so that a space-saving design, as is required for manyapplications such as traction applications, is not possible.Furthermore, the use of the physically large capacitors results in alarge amount of installation and maintenance effort.

DESCRIPTION OF THE INVENTION

One object of the invention is therefore to specify a converter circuitfor switching a large number of switching voltage levels, which storesas little electrical energy as possible during its operation, and whichcan be produced in a space-saving manner. This object is achieved by thefeatures of claim 1. Advantageous developments of the invention arespecified in the dependent claims.

The converter circuit according to the invention for switching a largenumber of switching voltage levels has n first switching groups whichare provided for each phase, with the n-th first switching group beingformed by a first power semiconductor switch and a second powersemiconductor switch, and the first first switching group to the(n−1)-th switching group each being formed by a first powersemiconductor switch and a second power semiconductor switch and by acapacitor which is connected to the first and second power semiconductorswitches, where, according to the invention, n≧1, and each of the nfirst switching groups when there are a plurality of first switchinggroups is connected in series with the respectively adjacent firstswitching group, and the first and the second power semiconductorswitches in the first first switching group are connected to oneanother. According to the invention, p second switching groups and pthird switching groups are provided, which are each formed by a firstpower semiconductor switch and a second power semiconductor switch andby a capacitor which is connected to the first and second powersemiconductor switches, where p≧1 and each of the p second switchinggroups when there are a plurality of second switching groups beingconnected in series with the respectively adjacent second switchinggroup. Each of the p third switching groups when there are a pluralityof third switching groups is connected in series with the respectivelyadjacent third switching group, and the first second switching group isconnected to the first power semiconductor switch in the n-th firstswitching group, and the first third switching group is connected to thesecond power semiconductor switch in the n-th first switching group.Furthermore, the capacitor in the p-th second switching group isconnected in series with the capacitor in the p-th third switchinggroup.

The p second switching groups and p third switching groups which areprovided, together with their connections as described above mean thatthe p second switching groups are involved, for example, only during thepositive half-cycle with respect to the phase output AC voltage, and thep third switching groups are involved only during the negativehalf-cycle, in operation of the converter circuit according to theinvention. It is thus advantageously possible to reduce the amount ofelectrical energy which is stored in the converter circuit, inparticular in the capacitors in the p second and third switching groups.Furthermore, the n first switching groups are used only for balancingthe phase output AC voltage, so that, when there are a plurality offirst switching groups, the capacitors in the n first switching groupsessentially carry no current in the balanced state, and thereforeessentially do not store any electrical energy either. The amount ofstored electrical energy in the converter circuit can thus be kept lowoverall, so that the capacitors in the converter circuit need bedesigned only for a small amount of electrical energy to be stored, thatis to say with respect to their withstand voltage and/or theircapacitance. Because of the small physical size of the capacitors, theconverter circuit requires very little space, thus advantageouslyallowing a space-saving design, as is required for many applications,for example for traction applications. Furthermore, the small physicalsize of the capacitors also advantageously makes it possible to keep theamount of installation and maintenance effort low.

These and further objects, advantages and features of the presentinvention will become evident from the following detailed description ofpreferred embodiments of the invention, in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures:

FIG. 1 a shows a first embodiment of a converter circuit according tothe invention,

FIG. 1 b shows a second embodiment of a converter circuit according tothe invention,

FIG. 1 c shows a third embodiment of a converter circuit according tothe invention,

FIG. 2 shows a fourth embodiment of the converter circuit according tothe invention,

FIG. 3 a shows a fifth embodiment of the converter circuit according tothe invention,

FIG. 3 b shows a sixth embodiment of the converter circuit according tothe invention, and

FIG. 4 shows a seventh embodiment of the converter circuit according tothe invention.

The reference symbols used in the drawing and their meanings are listedin a summarized form in the list of reference symbols. In principle,identical parts are provided with the same reference symbols in thefigures. The described embodiments represent examples of the subjectmatter of the invention, and have no restrictive effect.

WAYS TO IMPLEMENT THE INVENTION

FIG. 1 a shows a (in particular single-phase) first embodiment of aconverter circuit according to the invention for switching a largenumber of switching voltage levels. In this case, the converter circuithas n first switching groups 1.1, . . . , 1.n which are provided foreach phase R, Y, B, with the n-th first switching group 1.n being formedby a first power semiconductor switch 2 and a second power semiconductorswitch 3, and with the first first switching group 1.1 to the (n−1)-thswitching group 1.(n−1) in each case being formed by a first powersemiconductor switch 2 and a second power semiconductor switch 3, and bya capacitor 4 which is connected to the first and to the second powersemiconductor switch 2, 3, in which case, according to the invention,n≧1. Since, as can be seen from FIG. 1 a, each of the first switchinggroups 1, 1.1, . . . , 1.n represents a four-pole network, each of the nfirst switching groups 1.1, . . . , 1.n when there are a plurality offirst switching groups 1.1, . . . , 1.n is connected in series with therespectively adjacent first switching group 1.1, . . . , 1.n, that is tosay the n-th first switching group 1.n is connected in series with the(n−1)-th first switching group 1.(n−1), and the (n−1)-th first switchinggroup 1.(n−1) is connected in series with the (n−2)-th first switchinggroup 1.(n−2), etc. As can be seen from FIG. 1 a, the first and thesecond power semiconductor switches 2, 3 in the first first switchinggroup 1.1 are connected to one another. The junction point of the firstand of the second power semiconductor switches 2, 3 in the first firstswitching group 1.1 forms a phase connection, in particular for thephase R, as shown in FIG. 1 a.

According to the invention, and as shown in FIG. 1 a, p second switchinggroups 5.1, . . . , 5.p and p third switching groups 6.1, . . . , 6.pare now provided and are each formed by a first power semiconductorswitch 2 and a second power semiconductor switch 3, and by a capacitor 4which is connected to the first and second power semiconductor switches2, 3, where p≧1. Since, as shown in FIG. 2, each of the p secondswitching groups 5.1, . . . , 5.p and each of the p third switchinggroups 6.1, . . . , 6.p represents a four-pole network, each of the psecond switching groups 5.1, . . . , 5.p when there are a plurality ofsecond switching groups 5.1, . . . , 5.p is connected in series with therespectively adjacent second switching groups 5.1, . . . , 5.p, that isto say the p-th second switching group 5.p is connected in series withthe (p−1)-th second switching group 5.(p−1), and the (p−1)-th secondswitching group 5.(p−1) is connected in series with the (p−2)-th secondswitching group 5.(p−2), etc. Furthermore, as shown in FIG. 1 a, each ofthe p third switching group 6.1, . . . , 6.p when there are a pluralityof third switching groups 6.1, . . ., 6.p is connected in series withthe respectively adjacent third switching groups 6.1, . . ., 6.p, thatis to say the p-th third switching group 6.p is connected in series withthe (p−1)-th third switching group 6.(p−1), and the (p−1)-th thirdswitching group 6.(p−1) is connected in series with the (p−2)-th thirdswitching group 6.(p−2), etc.

Furthermore, the first second switching group 5.1 is connected to thefirst power semiconductor switch 2 in the n-th first switching group1.n, and the first third switching group 6.1 is connected to the secondpower semiconductor switch 3 in the n-th first switching group 1.n.Finally, the capacitor 4 in the p-th second switching group 5.p isconnected in series with the capacitor 4 in the p-th third switchinggroup 6.p. The p second switching groups 5.1, . . . , 5.p and p thirdswitching groups 6.1, . . . , 6.p that are provided and their describedconnections in each case between one another, to one another and to then-th first switching group 1.n mean that the p second switching groups5.1, . . . , 5.p are involved, for example, only in the positivehalf-cycle with respect to the phase output AC voltage, and the p thirdswitching groups 6.1, . . . , 6.p are involved only in the negativehalf-cycle with respect to the phase output AC voltage, in the operationof the converter circuit according to the invention. The amount ofelectrical energy which is stored in the converter circuit, inparticular in the capacitors 4 in the p second and third switchinggroups 5.1, . . . , 5.p; 6.1, . . . , 6.p can thus advantageously bereduced. Furthermore, the n first switching groups 1.1, . . . , 1.n areused only for balancing the phase output AC voltage, so that thecapacitors 4 in the n first switching groups 1.1, . . . , 1.nessentially carry no current when the phase output AC voltage is in thebalanced state, and essentially no electrical energy is stored in themeither. The amount of electrical energy stored in the converter circuitaccording to the invention can thus be kept low overall, so that thecapacitors 4 in the converter circuit need be designed only for a smallamount of electrical energy to be stored, that is to say with respect totheir withstand voltage and/or their capacitance. Because of the smallphysical size of the capacitors 4, the converter circuit requires aminimum amount of space, thus advantageously allowing a space-savingdesign, as is required for many applications, for example for tractionapplications. Furthermore, the small physical size of the capacitors 4also advantageously makes it possible to keep the installation andmaintenance effort low.

As shown in FIG. 1 a, a voltage limiting network 7, for example, isconnected in parallel with the first power semiconductor switch 2 in then-th first switching group 1.n, and a voltage limiting network 7 islikewise connected in parallel with the second power semiconductorswitch 3 in the n-th first switching group 1.n. The voltage limitingnetwork 7 can optionally be chosen and is advantageously used tostabilize the phase output voltage, in particular when the desired phaseoutput voltage is 0 V. The voltage limiting network 7 preferably has acapacitor or, as is shown in FIG. 1 a, a series circuit formed by aresistor with a capacitor. It is obvious to a person skilled in the artthat all the other first and second power semiconductor switches 2, 3 inthe first switching groups 1.1, . . . , 1.(n−1) as well as the secondand third switching groups 5.1, . . . , 5.p; 6.1, . . . , 6.p may alsohave a voltage limiting network 7, in particular of any type, and/or acurrent limiting network, in particular of any type.

FIG. 1 b shows a (in particular single-phase) second embodiment of theconverter circuit according to the invention for switching a largenumber of switching voltage levels. In contrast to the first embodimentshown in FIG. 1 a, the n-th first switching group 1.n in the secondembodiment as shown in FIG. 1 b has a capacitor 4 which is connected tothe first and second power semiconductor switches 2, 3 in the n-th firstswitching group 1.n, with the first second switching group 5.1 beingconnected to the capacitor 4 in the n-th first switching group 1.n, andthe first third switching group 6.1 being connected to the capacitor 4in the n-th first switching group 1.n. The capacitor 4 in the n-th firstswitching group 1.n advantageously results, particularly when thedesired phase output voltage is 0 V, in this phase output voltage beingstabilized, so that this can be achieved without any problems andwithout any disturbance effects. If the first embodiment as shown inFIG. 1 a is compared with the second embodiment as shown in FIG. 1 b,the capacitor 4 in the n-th first switching group 1.n can be chosenoptionally, and is used only for voltage limiting or for voltagestabilization, and thus cannot be regarded as a voltage source. It isalso feasible, although this is not shown in FIG. 1 a for the sake ofclarity, to provide a series circuit formed by the capacitor 4 with aresistor, instead of the capacitor 4 in the n-th first switching group1.n. It is self-evident that the capacitor 4 in the n-th first switchinggroup 1.n or the series circuit formed by the capacitor 4 with aresistor can be chosen optionally for all the described embodiments.

FIG. 1 c shows a (in particular single-phase) third embodiment of theconverter circuit according to the invention for switching a largenumber of switching voltage levels. In this case, the total number ofthe n first switching groups 1.1, . . . , 1.n is less than the totalnumber of the p second and third switching groups 5.1, . . . , 5.p; 6.1,. . . , 6.p. In FIG. 1 c, these are then n=1 first switching groups 1.1,1.2 and p=2 second switching groups 5.1, 5.2, as well as p=2 thirdswitching groups 6.1, 6.2. This advantageously means that fewer firstswitching groups 1.1, . . . , 1.n and thus fewer first and second powersemiconductor switches 2, 3 and fewer capacitors 4 are required, and thetotal space required for the converter circuit according to theinvention can thus be reduced further. The first and second powersemiconductor switches when there are n=1 first switching groups 1.1,1.2, as is shown by way of example in FIG. 1 c, are preferably eachformed by a high blocking-capability bidirectional power semiconductorswitch that is to say by a drivable high blocking-capability electroniccomponent which carries currents in only one direction, for example by agate turn-off thyristor (GTO thyristor) or an integrated thyristor witha commutated drive electrode (IGCT—integrated gate commutatedthyristor), and by a passive high blocking-capability electroniccomponent which is connected back-to-back in parallel with this, cannotbe driven and carries current in only one direction, for example by adiode.

FIG. 2 shows a (in particular single-phase) fourth embodiment of theconverter circuit according to the invention for switching a largenumber of switching voltage levels. In this case, the total number ofthe n first switching groups 1.1, . . . , 1.n corresponds to the totalnumber of the p second and third switching groups 5.1, . . . , 5.p; 6.1,. . . , 6.p. In FIG. 2, these are then n=2 first switching groups 1.1,1.2 and p=2 second switching groups 5.1, 5.2, as well as p=2 thirdswitching groups 6.1, 6.2. If the total number of the n first switchinggroups 1.1, . . . , 1.n corresponds to the total number of the p secondand third switching groups 5.1, . . . , 5.p; 6.1, . . . , 6.p then it isadvantageously in general possible to switch (2n+1) switching voltagelevels in the converter circuit according to the invention, that is tosay, if n=2 as shown in FIG. 2, five switching voltage levels can thenbe switched.

Furthermore, it is also feasible for the total number of the n firstswitching groups 1.1, . . . , 1.n to be greater than the total number ofthe p second and third switching groups 5.1, . . . , 5.p; 6.1, . . . ,6.p.

As shown in FIG. 1 a and FIG. 1 c, the first and second powersemiconductor switches 2, 3 in the first second switching group 5.1 areconnected to one another, with the junction point of the first andsecond power semiconductor switches 2, 3 in the first second switchinggroup 5.1 being connected to the first power semiconductor switch 2 inthe n-th first switching group 1.n. Furthermore, as shown in FIG. 1 aand FIG. 1 c, the first and second power semiconductor switches 2, 3 inthe first third switching group 6.1 are connected to one another, withthe junction point of the first and second power semiconductor switches2, 3 in the first third switching group 6.1 being connected to thesecond power semiconductor switch 3 in the n-th first switching group1.n.

As shown in FIG. 1 b, the first and second power semiconductor switches2, 3 in the first second switching group 5.1 are connected to oneanother, with the junction point of the first and second powersemiconductor switches 2, 3 in the first second switching group 5.1being connected to the junction point of the capacitor 4 in the n-thfirst switching group 1.n and the first power semiconductor switch 2 inthe n-th switching group 1.n. Furthermore, the first and second powersemiconductor switches 2, 3 in the first third switching group 6.1 areconnected to one another, with the junction point of the first andsecond power semiconductor switches 2, 3 in the first third switchinggroup 6.1 being connected to the junction point of the capacitor 4 inthe n-th first switching group 1.n and the second power semiconductorswitch 3 in the n-th first switching group 1.n.

The first power semiconductor switch 2 and the second powersemiconductor switch 3 in each switching group 1.1, . . . , 1.n; 5.1, .. . , 5.p; 6.1, . . . , 6.p are preferably each in the form of abidirectional power semiconductor switch, as in the case of theembodiment shown in FIG. 1 a, FIG. 1 b, FIG. 1 c and FIG. 2.

FIG. 3 a shows a (in particular single-phase) fifth embodiment of theconverter circuit according to the invention for switching a largenumber of switching voltage levels. As shown in FIG. 3 a, the firstpower semiconductor switch 2 in each first and in each second switchinggroup 1.1, . . . , 1.n; 5.1, . . . , 5.p is a bidirectional powersemiconductor switch. Furthermore, the second power semiconductor switch3 in each first switching group 1.1, . . . , 1.n and in each thirdswitching group 6.1, . . . , 6.p is a bidirectional power semiconductorswitch. In contrast to the embodiments shown in FIG. 1 a, FIG. 1 b, FIG.1 c and FIG. 2, the second power semiconductor switch 3 in each secondswitching group 5.1, . . . , 5.p and the first power semiconductorswitch 2 in each third switching group 6.1, . . . , 6.p is aunidirectional power semiconductor switch. This measure makes itpossible to further simplify the converter circuit according to theinvention.

FIG. 3 b shows a (in particular single-phase) sixth embodiment of theconverter circuit according to the invention for switching a largenumber of switching voltage levels. As shown in FIG. 3 b, the firstpower semiconductor switch 2 in each first and in each third switchinggroup 1.1, . . . , 1.n; 6.1, . . . , 6.p is a bidirectional powersemiconductor switch. Furthermore, the second power semiconductor switch3 in each first and in each second switching group 1.1, . . . , 1.n;5.1, . . . , 5.p is a bidirectional power semiconductor switch. Inaddition, the first power semiconductor switch 2 in each secondswitching group 5.1, . . . , 5.p and the second power semiconductorswitch 3 in each third switching group 6.1, . . . , 6.p is aunidirectional power semiconductor switch. In addition to theadvantages, as already mentioned for the fifth embodiment shown in FIG.3 a, of simplification of the converter circuit, the voltage across therespective capacitors 4 in each second and third switching group 5.1, .. . , 5.p; 6.1, . . . , 6.p in the sixth embodiment of the convertercircuit as shown in FIG. 3 b can also be set very easily, for example toa predetermined value, in particular by means of regulation.

FIG. 4 shows a (in particular single-phase) seventh embodiment of theconverter circuit according to the invention for switching a largenumber of switching voltage levels. In this case, the first powersemiconductor switch 2 and the second power semiconductor switch 3 ineach first switching group 1.1, . . . , 1.n is a bidirectional powersemiconductor switch. Furthermore, the first power semiconductor switch2 and the second power semiconductor switch 3 in each second switchinggroup 5.1, . . . , 5.p and in each third switching group 6.1, . . . ,6.p is a unidirectional power semiconductor switch. This measure resultsin the converter circuit according to the invention becoming arectifier, which is designed in a very simple and furthermorespace-saving manner, since it requires only a minimal number ofbidirectional power semiconductor switches.

Each of the bidirectional power semiconductor switches in theembodiments of the converter circuit according to the invention shown inFIG. 1 a to FIG. 4 is preferably formed by an electronic component whichcan be driven and carries current in only one direction, for example byan insulated gate bipolar transistor (IGBT) or, as shown in FIG. 1 c andas already mentioned, by a gate turn-off thyristor (GTO) or by anintegrated gate commutated thyristor (IGCT), and by a passive electroniccomponent, which is connected back-to-back in parallel with this, cannotbe driven and carries current in only one direction, for example by adiode. The first and second power semiconductor switches 2, 3, which arein the form of bidirectional power semiconductor switches as shown inFIG. 1 a, FIG. 1 b, FIG. 1 c and FIG. 2 are connected within therespective switching group 1.1, . . . , 1.n; 5.1, . . . , 5.p; 6.1, . .. , 6.p in such a way that they have opposite main controlled currentdirections, that is to say the electronic components which can be drivenand carry current in only one direction have opposite main controlledcurrent directions to one another. Furthermore, the passive electroniccomponents which cannot be driven and carry current in only onedirection in the first and second power semiconductor switches 2, 3, asshown in FIG. 1 a, FIG. 1 b, FIG. 1 c and FIG. 2 are connected withinthe respective switching group 1.1, . . . , 1.n; 5.1, . . . , 5.p; 6.1,. . . , 6.p in such a way that they have a mutually opposite controlledcurrent direction.

Furthermore, each of the unidirectional power semiconductor switches arebased on the embodiments of the converter circuit according to theinvention as shown in FIGS. 3 a, b and FIG. 4 is preferably formed by apassive electronic component which cannot be driven and carries currentin only one direction, for example by a diode. As already mentioned, theconverter circuit according to the invention and as shown in FIGS. 3 a,b and FIG. 4 can be further simplified by this measure because fewerelectronic components which can be driven and carry current in only onedirection are required, and the drive complexity can thus besignificantly reduced. The first and second power semiconductor switches2, 3 which are in the form of bidirectional power semiconductor switchesas shown in FIGS. 3 a, b and FIG. 4 are connected within the respectivefirst switching groups 1.1, . . . , 1.n in such a way that they have anopposite controlled main current direction, that is to say theelectronic components which can be driven and carry current in only onedirection have a mutually opposite controlled main current direction.Furthermore, as shown in FIGS. 3 a, b for the respective second andthird switching groups 5.1, . . . , 5.p; 6.1, . . . , 6.p, the passiveelectronic component which cannot be driven and carries current in onlyone direction in the first and second power semiconductor switches 2, 3and the electronic component which can be driven and carries current inonly one direction in the first and second power semiconductor switches2, 3 are connected within the respective second and third switchinggroups 5.1, . . . , 5.p; 6.1, . . . , 6.p in such a way that they have amutually opposite current direction. Finally, the first and second powersemiconductor switches 2, 3, which are in the form of unidirectionalpower semiconductor switches as shown in FIG. 4, within the respectivesecond and third switching groups 5.1, . . . , 5.p; 6.1, . . . , 6.p areconnected in such a way that they have a mutually opposite currentdirection.

Furthermore, it has been found to be very advantageous in the case ofthe n first switching groups 1.1, . . . , 1.n to integrate the two firstpower semiconductor switches 2 in respectively adjacent first switchinggroups 1.1, . . . , 1.n in a module, that is to say when there are aplurality of first switching groups 1.1, . . . , 1.n, the first powersemiconductor switch 2 in the n-th first switching group 1.n and thefirst power semiconductor switch 2 in the (n−2)-th first switching group1.(n−1) are integrated in a module, and the first power semiconductorswitch 2 in the (n−1)-th first switching group 1.(n−1) and the firstpower semiconductor switch 2 in the (n−2)-th first switching group1.(n−2) are integrated in a module, etc. Furthermore, it has been foundto be advantageous, in the case of the n first switching groups 1.1, . .. , 1.n, for the two second power semiconductor switches 3 inrespectively adjacent first switching groups 1.1, . . . , 1.n to beintegrated in a module, that is to say, when there are a plurality offirst switching groups 1.1, . . . , 1.n, the second power semiconductorswitch 3 in the n-th first switching group 1.n and the second powersemiconductor switch 3 in the (n−1)-th first switching group 1.(n−1) areintegrated in a module, and the second power semiconductor switch 3 inthe (n−1)-th first switching group 1.(n−1) and the second powersemiconductor switch 3 in the (n−2)-th first switching group 1.(n−2) areintegrated in a module, etc. Modules such as these are normally standardhalf-bridge modules and are accordingly of simple design, are notsusceptible to faults, and are thus cost-effective. Furthermore, whenthere are a plurality of second switching groups 5.1, . . . , 5.p it hasbeen found to be advantageous, in the case of the p second switchinggroups 5.1, . . . , 5.p, for the two first power semiconductor switches2 in respectively adjacent second switching groups 5.1, . . . , 5.p tobe integrated in a module, and for the two second power semiconductorswitches 3 in respectively adjacent second switching groups 5.1, . . . ,5.p to be integrated in a module, in the manner described in detailabove for the first switching groups 1.1, . . . , 1.n. Furthermore, whenthere are a plurality of third switching groups 6.1, . . . , 6.p, it hasbeen found to be advantageous, in the case of the p third switchinggroups 6.1, . . . , 6.p, for the two first power semiconductor switches2 in respectively adjacent third switching groups 6.1, . . . , 6.p to beintegrated in a module, and for the two second power semiconductorswitches 3 in respectively adjacent third switching groups 6.1, . . . ,6.p to be integrated in a module, in the manner described in detailabove for the first switching groups 1.1, . . . , 1.n. It isself-evident that the integration, as explained in detail above, of therespective first and second power semiconductor switches 2, 3 applies toall of the embodiments of the converter circuit according to theinvention as shown in FIG. 1 a to FIG. 4.

However, it is also feasible, in the case of the n first switchinggroups 1.1, . . . , 1.n, in the case of the p second and third switchinggroups 5.1, . . . , 5.p; 6.1, . . . , 6.p to in each case integrate thefirst power semiconductor switch 2 and the second power semiconductorswitch 3 in a module. As already mentioned, modules such as these arenormally standard half-bridge modules and are accordingly of simpledesign, are not susceptible to faults, and are thus cost-effective. Inthis case as well, it is self-evident that the integration, as explainedin detail above, of the respective first and second power semiconductorswitches 2, 3 applies to all the embodiments of the converter circuitaccording to the invention as shown in FIG. 1 a to FIG. 4.

In the case of a converter circuit according to the invention that isintended to be provided for a polyphase application, the p-th secondswitching groups 5.p for the phases R, Y, B are preferably connected inparallel, and the p-th third switching groups 6.p for the phases R, Y, Bare connected in parallel with one another. The respective connectionsare made to the capacitors 4 in the respective p-th second switchinggroups 5.p, and to the capacitors 4 in the respective p-th thirdswitching groups 6.p, respectively.

In order advantageously to allow space to be saved in the case of apolyphase converter circuit, the capacitors 4 in the p-th secondswitching groups 5.p for the phases R, Y, B are preferably combined toform one capacitor. Furthermore, the capacitors 4 in the p-th thirdswitching groups 6.b for the phases R, Y, B are preferably likewisecombined to form one capacitor.

Overall, the converter circuit according to the invention for switchinga large number of switching voltage levels thus represents a solutionwhich is characterized by storing only a small amount of electricalenergy during its operation and by its space-saving design, and thusrepresents a solution which is uncomplicated, robust and is notsusceptible to defects.

LIST OF REFERENCE SYMBOLS

-   1.1, . . . , 1.n First switching groups-   2 First power semiconductor switch-   3 Second power semiconductor switch-   4 Capacitor-   5.1, . . . , 5.p Second switching groups-   6.1, . . . , 6.p Third switching groups-   7 Voltage limiting network

1. A converter circuit for switching a large number of switching voltagelevels, having n first switching groups which are provided for eachphase (R, Y, B), with the n-th first switching group being formed by afirst power semiconductor switch and a second power semiconductorswitch, and with the first first switching group to the (n−1)-thswitching group each being formed by a first power semiconductor switchand a second power semiconductor switch and by a capacitor, which isconnected to the first and second power semiconductor switches, witheach of the n first switching groups being connected in series to therespectively adjacent first switching group, and with the first and thesecond power semiconductor switches in the first first switching groupbeing connected to one another, wherein n≧1 and p second switchinggroups and p third switching groups are provided, which are each formedby a first power semiconductor switch and a second power semiconductorswitch and by a capacitor which is connected to the first and secondpower semiconductor switches, where p≧1 and each of the p secondswitching groups is connected in series with the respectively adjacentsecond switching group, and each of the p third switching groups isconnected in series with the respectively adjacent third switchinggroup, and the first second switching group is connected to the firstpower semiconductor switch in the n-th first switching group, and thefirst third switching group is connected to the second powersemiconductor switch in the n-th first switching group, and in that thecapacitor in the p-th second switching group is connected in series withthe capacitor in the p-th third switching group.
 2. The convertercircuit as claimed in claim 1, wherein a voltage limiting network isconnected in parallel with the first power semiconductor switch in then-th first switching group, and in that a voltage limiting network isconnected in parallel with the second power semiconductor switch in then-th first switching group.
 3. The converter circuit as claimed in claim2, wherein the voltage limiting network (7) has a capacitor.
 4. Theconverter circuit as claimed in claim 2, wherein the voltage limitingnetwork has a series circuit formed by a resistor with a capacitor. 5.The converter circuit as claimed in claim 1, wherein the n-th firstswitching group has a capacitor which is connected to the first andsecond power semiconductor switches in the n-th first switching group,with the first second switching group being connected to the capacitorin the n-th first switching group, and with the first third switchinggroup being connected to the capacitor in the n-th first switchinggroup.
 6. The converter circuit as claimed in claim 1, wherein the firstand second power semiconductor switches in the first second switchinggroup are connected to one another, with the junction point of the firstand second power semiconductor switches in the first second switchinggroup being connected to the first power semiconductor switch in then-th first switching group, and in that the first and second powersemiconductor switches in the first third switching group are connectedto one another, with the junction point of the first and second powersemiconductor switches in the first third switching group beingconnected to the second power semiconductor switch in the n-th firstswitching group.
 7. The converter circuit as claimed in claim 5, whereinthe first and second power semiconductor switches in the first secondswitching group are connected to one another, with the junction point ofthe first and second power semiconductor switches in the first secondswitching group being connected to the junction point of the capacitorin the n-th first switching group and the first power semiconductorswitch in the n-th first switching group, and in that the first andsecond power semiconductor switches in the first third switching groupare connected to one another, with the junction point of the first andsecond power semiconductor switches in the first third switching groupbeing connected to the junction point of the capacitor in the n-th firstswitching group and the second power semiconductor switch in the n-thfirst switching group.
 8. The converter circuit as claimed in claim 1,wherein the total number of then first switching groups corresponds tothe total number of the p second and third switching groups.
 9. Theconverter circuit as claimed in claim 1, wherein the total number of then first switching groups is less than the total number of the p secondand third switching groups.
 10. The converter circuit as claimed inclaim 1, wherein the total number of the n first switching groups isgreater than the total number of the p second and third switchinggroups.
 11. The converter circuit as claimed in claim 1, wherein thefirst power semiconductor switch and the second power semiconductorswitch in each switching group are in each case in the form of abidirectional power semiconductor switch.
 12. The converter circuit asclaimed in claim 1, wherein the first power semiconductor switch in eachfirst and in each second switching group is a bidirectional powersemiconductor switch, wherein the second power semiconductor switch ineach first and in each third switching group is a bidirectional powersemiconductor switch, and wherein the second power semiconductor switchin each second switching group and the first power semiconductor switchin each third switching group are in each case in the form of aunidirectional power semiconductor switch.
 13. The converter circuit asclaimed in claim 1, wherein the first power semiconductor switch in eachfirst and in each third switching group is a bidirectional powersemiconductor switch, wherein the second power semiconductor switch ineach first and in each second switching group is a bidirectional powersemiconductor switch, and wherein the first power semiconductor switchin each second switching group and the second power semiconductor switchin each third switching group is a unidirectional power semiconductorswitch.
 14. The converter circuit as claimed in claim 1, wherein thefirst power semiconductor switch and the second power semiconductorswitch in each first switching group are in each case in the form of abidirectional power semiconductor switch, and wherein the first powersemiconductor switch and the second power semiconductor switch in eachsecond switching group and in each third switching group are in eachcase in the form of a unidirectional power semiconductor switch.
 15. Theconverter circuit as claimed in claim 11, wherein the bidirectionalpower semiconductor switch is formed by an electronic component whichcan be driven and carries current in only one direction, and by apassive electronic component which is connected back-to-back in parallelwith this, cannot be driven and carries current in only one direction.16. The converter circuit as claimed in claim 12, wherein theunidirectional power semiconductor switch is formed by a passiveelectronic component which cannot be driven and carries current in onlyone direction.
 17. The converter circuit as claimed in claim 1, wherein,in the case of the n first switching groups, the two first powersemiconductor switches in respectively adjacent first switching groupsare integrated in a module, and the two second power semiconductorswitches in respectively adjacent first switching groups are integratedin a module.
 18. The converter circuit as claimed in claim 17, wherein,in the case of the p second switching groups, the two first powersemiconductor switches in respectively adjacent second switching groupsare integrated in a module, and the two second power semiconductorswitches in respectively adjacent second switching groups are integratedin a module, and wherein, in the case of the p third switching groups,the two first power semiconductor switches in respectively adjacentthird switching groups are integrated in a module, and the two secondpower semiconductor switches in respectively adjacent third switchinggroups are integrated in a module.
 19. The converter circuit as claimedin claim 1, wherein, in the case of the n first switching groups and inthe case of the p second and third switching groups, the first powersemiconductor switch and the second power semiconductor switch are ineach case integrated in a module.
 20. The converter circuit as claimedin claim 1, wherein, if there are a plurality of phases (R, Y, B), thep-th second switching groups for the phases (R, Y, B) are connected inparallel with one another, and the p-th third switching groups for thephases (R, Y, B) are connected in parallel with one another.
 21. Theconverter circuit as claimed in claim 20, wherein the capacitors in thep-th second switching groups for the phases (R, Y, B) are combined toform one capacitor, and wherein the capacitors in the p-th thirdswitching groups for the phases (R Y, B) are combined to form onecapacitor.